1. Procedure for Drawing Flow Nets

At every point (x,z) where there is flow, there will be a value of head h(x,z). In order to represent these values, contours of equal head are drawn.

In fluid mechanics and hydrogeology, 'head' refers to the height of the water column and indicates the energy level of water at a specific point. This head can vary depending on the location (x, z) within the soil. To visualize the variations in this head, we draw contours which connect points of equal head. This helps in understanding how water flows through the soil matrix.

Think of a landscape with hills and valleys. Just like the height of a hill represents a higher head compared to a valley, the head in soil indicates how 'high' the water would rise at different points. Drawing contours of equal height (head) helps us visualize how water would flow from higher areas to lower areas.

A flow net is to be drawn by trial and error. For a given set of boundary conditions, the flow net will remain the same even if the direction of flow is reversed. Flow nets are constructed such that the head lost between successive equipotential lines is the same, say ∆h.

Drawing flow nets requires a trial-and-error technique, meaning we may need to adjust our net several times to find a configuration that fits the boundary conditions correctly. The flow net displays lines of flow and equipotential lines. Importantly, the head, or pressure exerted by the water, should decrease consistently (by ∆h) between these lines, allowing us to draw accurate representations of how water flows.

Consider building a racecourse on varying terrains. Just as you would try different paths to find one that works best while ensuring the slopes are consistent for racers, constructing flow nets involves testing until you find the perfect lines that illustrate how water moves in the soil.

The steps of construction are:

1. Mark all boundary conditions, and draw the flow cross section to some convenient scale.
2. Draw a coarse net which is consistent with the boundary conditions and which has orthogonal equipotential and flow lines. As it is usually easier to visualize the pattern of flow, start by drawing the flow lines first.
3. Modify the mesh such that it meets the conditions outlined above and the fields between adjacent flow lines and equipotential lines are 'square'.
4. Refine the flow net by repeating step 3.

To construct a flow net:

1. First, identify and mark the boundaries where water enters or exits the soil. This provides guidelines for your net.
2. Next, start with a rough drawing of the flow lines, which shows the path water will follow. It’s important to make sure that these flow lines are perpendicular (orthogonal) to the equipotential lines.
3. Adjust the grid or mesh to ensure that areas between the flow lines and equipotential lines remain as 'square' shapes.
4. Finally, refine your drawing by revisiting the mesh and making necessary adjustments to ensure accuracy.

Imagine using a map to chart a course for a bike race. You begin by marking where the race starts and ends (the boundary conditions). Then, you draw the route with clear paths (flow lines) while ensuring paths intersect at right angles (equipotential lines). By adjusting your paths as needed, you ensure racers can navigate smoothly, similar to refining your flow net.

The most common boundary conditions are: (a) A submerged permeable soil boundary is an equipotential line. This could have been determined by considering imaginary standpipes placed at the soil boundary, as for every point the water level in the standpipe would be the same as the water level. (Such a boundary is marked as CD and EF in the following figure.) (b) The boundary between permeable and impermeable soil materials is a flow line (This is marked as AB in the same figure). (c) Equipotential lines intersecting a phreatic surface do so at equal vertical intervals.

When drawing flow nets, it's important to understand the common conditions at the boundaries: (a) A submerged permeable boundary, where water can flow freely, acts as an equipotential line, meaning all points have the same head (pressure). (b) Conversely, a boundary between permeable (water can flow through) and impermeable (water cannot flow through) materials is represented as a flow line, indicating where water is moving. (c) Lastly, equipotential lines intersecting a phreatic surface (the level of groundwater) do so at regular (equal) intervals, which helps maintain the consistency of the flow representation.

Think of a water bottle filled halfway with different materials at the bottom. The water's surface represents the phreatic surface and the top of the different materials creates flow lines. Where the water can flow easily (permeable) is like an open pathway, while areas where it cannot flow act like walls obstructing the water.

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